Can someone explain Zed's ult to me?

why sometimes it does like no damage and sometimes it instagibs me---Hey what's going on in this thread http://i.imgur.com/9VkxI.gifMy friend's YouTubes. She's got LP's and stuff. http://www.youtube.com/user/ratbirdchampion

Does extra damage based on the damage done while he ulted. So if he does no damage to you while he ults you it won't do a lot, but if he does a crap ton then it will gib you bro.---The problem here is, Darius bursts like Akali, takes damage like Nasus, and snowballs like Jesus Christ.LoL IGN: Kamon Race

it has a flat damage, and it also deals an additional % of the total amount of damage he deals to you during its duration (dealt when the mark detonates)---XBL: iiTryHaard LoL IGN: PintbombYes, ii do try hard http://www.lowbird.com/data/images/2012/08/imgur-dofor.jpg

"After 3 seconds, the mark will detonate, dealing physical damage equal to 100% of Zed's attack damage plus a percentage of all magic and physical damage dealt to the target by Zed and his shadows while the mark was active. "

"After 3 seconds, the mark will detonate, dealing physical damage equal to 100% of Zed's attack damage plus a percentage of all magic and physical damage dealt to the target by Zed and his shadows while the mark was active. "

Lolwiki

Real Question is why it lists Magic Damage since Zed does none

Item damage counts.Wit's End, Gunblade, etc.---On the field of honor, the gods bearing witness to my glory, I will pull your entrails through your eyes and use them as reins to ride your soul to hades below!